This invention generally relates to insulation panels for low temperature liquified gas cryogenic freezing tunnels and spirals. Cryogenic freezing tunnels are used for freezing food products. A tunnel is composed of elongated insulted panels and a conveyer belt mounted inside and to the floor of the tunnel. Food products which move through the tunnel on the conveyer belt are quickly frozen by contact with liquid nitrogen or other cryogenic liquid sprayed from over head spray headers mounted in the tunnel as the food products pass through the tunnel on a conveyer belt.
The spray headers are located on the inside ceiling of the tunnel. These spray headers release liquid nitrogen N.sub.2 or other cryogenic media onto the food products passing below on the conveyer belt. The liquid nitrogen contacting the food quickly freezes the food. The remainder of the liquid nitrogen falls onto collection pans located below the conveyer belt and vaporizes.
The walls of the tunnel are made from insulated panels. The insulted panels of the tunnel are made from multiple layers of material. These layers of materials include an outside layer of metal, a thick layer of foam insulation adjacent the outside metal layer, a layer of plywood abutting the foam insulation, and an inside metal skin abutting the plywood. The conveyer belt has conveyer belt supports mounted onto the inside skin of the floor of the tunnel. The collection pans are mounted to the floor of the insulated tunnel. This construction allows food to be frozen quickly as the food travels on the conveyer belt through the tunnel.
However, in operation several problems may arise. Air tends to accumulate in void spaces between the plywood and inside metal skin due to openings in welded seams or cracks. The accumulation of the air has the potential of condensing into oxygen due to the temperature differentials between the inside of the freezer tunnel and the void spaces. The moisture in the air condenses and transforms into ice crystals which steadily expand in size.
When air having moisture in it infiltrates the void spaces in the panel, two different conditions occur. First, the moisture condenses due to the extreme difference in temperature and transforms into ice, constantly increasing in size until the panels buckle, damaging the weld seams thereby, allowing more air to enter the void spaces causing even larger blocks of ice to form. This cycle repeats itself causing further damage to the freezer tunnel. This expansion of H.sub.2 O causes ice build-up between the plywood and inside metal skin, therefore causing bulging of the external surfaces of the inside metal skin, as well as cracking of the welded seams. Since both the collection pans and support brackets are mounted on the inside metal skin, the collection pans and support brackets are forced upward by the bulging of the external surfaces of the inside metal skin, thereby, forcing the collection pans and conveyer belt supports upward into the conveyer belt causing belt damage, breakage of conveyer belt supporting frames, thereby, resulting shutdown of the cryogenic freezer tunnel.
The second condition can occur when the internal freezer temperature approaches cryogenic temperatures of-280.degree. F. to -320.degree. F., thereby, causing air in the void spaces to separate into oxygen and nitrogen. This concentration of oxygen in the void spaces can contribute to the combustion of the insulation.
In accordance with the present invention, the problems of ice build-up and bulging of the inside metal skin can be alleviated by connecting the outer metal skin and the inner surface of the freezer with cryogenic tie pins. The tie pins act to keep the inner metal skin compressed against the wooden layers in the panels, and thereby, limits the number of void spaces between the inner surface and wooden layer. Consequently, this limits ice build up beneath the inner surface which causes bulging and consequential break down of the conveyer belt. Additionally, seams created by connecting the panels together are welded together to prevent breathing of air within the insulated chamber, thereby, further minimizing condensing of moisture. Also, the problem of the combustion of foam insulation is minimized. The cryogenic tie pins have low thermal conductivity. The low conductivity of the tie pins minimizes heat loss and formation of ice on the ends of the tie pins.